Guided-mode resonance is a phenomenon wherein the guided modes of an optical waveguide can be excited and simultaneously extracted by the introduction of a phase-matching element, such as a diffraction grating or prism.
Grating couplers are known in the art. Such coupler may comprise a diffraction grating in a region on top of or below a waveguide, e.g. an integrated waveguide in a photonic integrated circuit that may for example be composed of a transparent dielectric. Thus, off-resonance light incident on the grating may behave almost the same as it would if it was incident in an area where there is no grating, while for specific combinations of incident angles and light frequency, resonance occurs, allowing the grating to couple light into a guided mode of the waveguide.